An ultrasonic diagnostic apparatus is a medical imaging equipment, which non-invasively obtains tomograms of in vivo soft tissue from the body surface using the ultrasonic pulse reflection method. This ultrasonic diagnostic apparatus is characterized by being small-sized, inexpensive, and highly safe due to no need for exposure to X-rays, compared to other medical imaging equipment, and further, is characterized by enabling blood flow imaging via application of the Doppler effect. Ultrasonic diagnostic apparatuses have been widely used in the circulatory system (coronary artery), the digestive system (stomach and intestines), internal medicine (liver, pancreas, and spleen), the urinary system (kidney and bladder), as well as obstetrics and gynecology. Since ultrasound probes, commonly used in such medical ultrasonic diagnostic apparatuses, transmit and receive ultrasonic waves of high sensitivity and high resolution, wherefore piezoelectric effects exhibited in piezoelectric inorganic materials are generally utilized. In this case, single-type transducers, being a monotype, or array-type transducer formed by positioning a plurality of transducers two-dimensionally are commonly employed to obtain vibration modes for transmitting piezoelectric elements. The array-type transducers capable of obtaining highly detailed images have become widespread in medical imaging applied to diagnostic tests.
On the other hand, harmonic imaging diagnosis using high harmonic signals is becoming the standard diagnostic modality since it is possible to obtain clear diagnostic images, which are not obtainable by conventional B-mode diagnosis. Harmonic imaging has many advantages in comparison with fundamental wave imaging, such as: high contrast resolution stemming from an excellent S/N ratio due to a low sidelobe level, high resolution in the horizontal direction stemming from a narrow beam-width due to high frequency, no occurrence of multiple reflection due to low sound pressure and small sound pressure fluctuation over a short distance, and realization of high depth-speed due to attenuation comparable to that of a fundamental wave at the focal point or deeper, compared to ultrasonic imaging methods using fundamental wave of a high harmonic frequency.
A piezoelectric vibrator is utilized as a specific structure of an array-type ultrasound probe for harmonic imaging, wherein each of the vibrator elements forming an array is broadband-monolithic. A method of transmitting a fundamental wave in the frequency range on the low frequency side and of receiving a high harmonic wave in the frequency range on the high frequency side is commonly utilized, based on the broadband performance of the piezoelectric vibrator. In such a situation, the following technology for increasing sensitivity of the conventional ultrasound probe is known: “a technology for aiming to increase sensitivity by means of vibrating vertically each of fine columnar piezoelectric elements (columnar inorganic materials), wherein vibrators formed by packing the fine columnar piezoelectric elements by use of, for example, an organic compound such as an epoxy resin are employed as ultrasonic transmitting and receiving elements” (refer to Patent Document 1).
Preferably, a narrow-band ultrasonic wave is utilized to prevent the overlap between the spectrums of an ultrasonic wave to transmit a fundamental wave and an ultrasonic wave to receive a high harmonic wave. The narrow-band ultrasonic wave is normally an ultrasonic pulse signal with a long tail, resulting in negatively affecting the resolution in the depth direction.
As a specific structure of other array-type ultrasound probes for harmonic imaging, for example, a separate-type transmitting and receiving transducer has been proposed, wherein each of the transmitting and receiving piezoelectric vibrators is positioned separately, (refer to Patent Documents 2 and 3). Further, it has been proposed that a second piezoelectric layer to receive an ultrasonic wave of a central frequency of f2=2f1 is piled on a first piezoelectric layer, wherein the first piezoelectric layer is a piezoelectric layer to transmit and receive an ultrasonic wave with a central frequency of f1, composed of an array of plural first piezoelectric elements with a first acoustic impedance, and the second piezoelectric layer incorporates an array of plural second piezoelectric elements exhibiting a second acoustic impedance (refer to Patent Document 3); however, sufficient sensitivity has not yet been obtained.
Further, the following method is applied to increase sensitivity of ultrasonic transmitting and receiving elements: transmitting sensitivity is increased by improving electric matching conditions of piezoelectric inorganic elements with the driving circuit by decreasing the apparent impedance using laminated piezoelectric inorganic elements; and by making large distortion by increasing the electric field intensity applied to the aforesaid elements (refer to Patent Document 5). However, although transmitting sensitivity in a laminated structure is increased according to the number of laminated layers, receiving sensitivity is inversely proportional to the number of the laminated layers, resulting in a disadvantage for harmonic imaging.
The composite vibrators described in above Patent Document 1 are prepared by forming a columnar structure by cutting an inorganic piezoelectric material using a cutter such as a dicer, followed by filling the cut grooves with an organic material such as an epoxy resin. Also in an array-type transducer, a cutter such as a dicer is employed to divide to form the channels.
However, since transmitting and receiving frequencies of an ultrasound probe depend on thickness of the inorganic piezoelectric materials, the higher the frequencies are, the smaller the size of the columnar structure or the pitch of the channel becomes. Therefore, the number of machining processes using a cutter such as a dicer increases, and the mechanical strength of the inorganic piezoelectric materials decreases, whereby it becomes difficult to ignore the degradation in characteristic caused by breakage, as well as heat and distortion during machining, resulting in causing problems of being liable to lower the yield in producing vibrators or probes, and to degrade the performance. Further, in order to prepare a transmitting piezoelectric element compatible with a desired frequency, it is necessary to grind and polish both end-surfaces thereof, which results in an increase of production cost. Thus, there have been problems to be solved in producing an ultrasound probe using an inorganic material, compared to when organic materials are used.
In order to solve the above problems on the conventional technology, there has been proposed a method in which an inorganic material is used as the transmitting piezoelectric element and an organic materials is used as the high sensitivity receiving piezoelectric element, whereby a high sensitivity ultrasound probe is obtained, specifically when an array-type ultrasound probe in which the transmitting operation and the receiving operation are divided is employed. In order to add piezoelectric characteristics to the organic materials, an effective polarization treatment such as high-voltage treatment or high-voltage corona discharge treatment is needed, however, there has been a problem that it is difficult to apply a high voltage to such an organic material due to breakage or dielectric breakdown of the organic materials (organic film layers). A method has been proposed in Patent document 4 to solve this problem, namely, the above organic porous material is sandwiched between dielectric substances, or dielectric oil is injected into the porous holes during polarization treatment of the organic material. The breakdown of the organic material can be avoided by the protection by sandwiching the organic material using the dielectric substances, however, the aforesaid protection causes a decrease in efficiency during polarization treatment, resulting in decrease in sensitivity. Further, use of dielectric oil causes a problem in that it is necessary to wipe off the oil, in order not to cause failure in printing electrodes on the wiped surface during the electrode mounting process. In other words, there has been a problem of requiring extra cost for cleaning.    (Patent Document 1) Japanese Patent Application Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 63-252140    (Patent Document 2) JP-A No. 8-187245    (Patent Document 3) JP-A No. 11-276478    (Patent Document 4) JP-A No. 6-342947    (Patent Document 5) JP-A No. 2005-235878